柴北缘小赛什腾山片麻状花岗岩的成因:来自地球化学、锆石U-Pb年代学及Hf同位素约束

Petrogenesis of the Gneissic Granite in Xiaosaishiteng Mountain,Northern Qaidam:Constraint from Geochemistry,Zircon U-Pb Geochronology and Hf Isotopes

小赛什腾山位于柴北缘的西北部,笔者对小赛什腾山片麻状花岗岩进行了岩相学、锆石U-Pb年代学、全岩地球化学分析和锆石Hf同位素研究。对片麻状花岗岩进行LA-ICP-MS锆石U-Pb年龄测定,获得年龄为(422.1±1.6)Ma,指示该岩体为晚志留世岩浆活动产物。片麻状花岗岩地球化学成分显示高硅(SiO_(2)为70.16%~72.43%)、高碱(Na_(2)O+K_(2)O为7.20%~9.04%)和较高的K_(2)O/Na_(2)O值(>1),低镁(MgO为0.82%~1.29%)、磷(P_(2)O_(5)为0.09%~0.13%)及钛(TiO_(2)为0.3%~0.49%),铝饱和指数A/CNK值大于1,里特曼指数小于3.3,属于过铝质和高钾钙碱性系列;并且它们稀土元素总量高,轻重稀土分馏程度较高,具有弱的负Eu异常(δEu为0.73~0.96)。球粒陨石标准化配分曲线模式图上曲线一致向右倾斜,原始地幔标准化微量元素蛛网图上样品都具有相似的配分型式,具有富集大离子亲石元素Rb和亏损高场强元Nb、Ti的地球化学特征。全岩锆石饱和温度为721~752℃,平均值为739℃,属于I型花岗岩。锆石Hf同位素研究表明,片麻状花岗岩的ε_(Hf)(t)均为负值(-6.47~-12.85),模式年龄T_(DM2)平均值是2030Ma,指示小赛什腾山片麻状花岗岩的源区物质有中元古代的古老地壳物质混入,该花岗岩形成时有流体加入,由地壳基底物质重熔形成。综合分析后认为小赛什腾山片麻状花岗岩形成于后碰撞环境。

The Xiaosaishiteng Moutain is located in the Northwest Qaidam tectonic belt.A integrated study of petrography,zircon U-Pb chronology,geochemistry and zircon Hf isotopes in gneissic granite of the Xiaosaishiteng Moutain were conducted in this paper.The LA-ICP-MS ziron U-Pb data results indicate that the gneissic granites,which have high SiO_(2)(70.16%~72.43%),total alkali(Na_(2)O+K_(2)O=7.20%~9.04%)with K_(2)O/Na_(2)O>1,and low MgO(0.82%~1.29%),P_(2)O_(5)(0.09%~0.13%)and TiO_(2)(0.3%~0.49%),were formed in the late Silurian(422.1±1.6)Ma.Their A/CNK ratios are larger than 1,the Ritman index mainly are less than 3.3,therefore these gneissic granites are belong to peraluminous and high-K calc-alkaline series.They have relatively high REEs contents,enrichment in light REE with depletion in heavy REE,and exhibit weak negative Eu anomalies(δEu=0.73~0.96).The chondrite normalized REE patterns show dipped-right pattern,the primitive mantle Normalized trace elements pider diagram have the similar pattern.The gneissic granites strongly enriched in LILE and depletion of HFSE setting,such as Nb and Ti.The zircon saturation temperature is 721~725℃,the average is 739℃,belongs to I-type granite.The results of zircon Hf isotope shows that theε_(Hf)(t)of gneissic granite are negative(-6.47~12.85),and the crustal model age(2030Ma)indicates that the source rocks of the Xiaosaishiteng moutain gneissic granite was mixed with the granitic magma and the ancient crustal material of the Middle Proterozoic.The granite was formed when fluid was added,which from the remelting of crust.Comprehensive analysis shows that the granites was formed in the post-collision environment.